The present invention relates to an oxide superconducting multilayer structure made with superconductive and non-superconductive thin films and constituted by a combination of material groups for making interfaces among the thin film layers strain free and a Josephson device used in a high speed superconductive circuit, a highly sensitive magnetic field sensor or an electromagnetic wave detector, particularly to a technology effective in applying to an integrated circuit in an electronic instrument of communication and information processing.
A multilayered film structure constituted by a superconductor and a normally conductive metal is an important composite material group in view of achieving large current application or high frequency current application since the magnetic field penetration length can effectively be utilized. Therefore, an investigation is widely carried out on the multilayered structure fabrication process also in a metal superconductor group having a low critical temperature Tc. In addition thereto, in respect of an oxide superconductor having an especially high critical temperature Tc, the crystal structure per se is in a layered form and many researches have been carried out with respect to a multilayered film also in view of studying the mechanism of superconductivity.
Further, this technology is expected to be a basis in realizing a tunnel junction of a laminated layer type which becomes a basic device in electronic application. Incidentally, it is advantageous in view of crystal growth to combine the same kinds of materials in realizing a multilayered thin film. Therefore, in respect of a superconductor having a 123 structure having a representative example of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (.delta.; Oxygen depletion amount), PrBa.sub.2 Cu.sub.3 O.sub.7-.delta. showing no superconductivity irrespective of the fact that the compound is provided with approximately the same crystal structure, is selected and many fabrication results have been reported. However, in almost all of the conventional cases, YBa.sub.2 Cu.sub.3 O.sub.7-.delta. of which high quality thin film stacking process has been established as a superconductor layer was selected and fabricated.
The layer by layer technology of an oxide superconducting multilayered thin film structure concerning the above-mentioned conventional technology is disclosed in a paper by Hideomi Koinuma, Mamoru Yoshimoto, "Applied Physics", volume 60, No. 5 (1991), pp. 433-442.
Further, the technology of laminating high quality YBa.sub.2 Cu.sub.3 O.sub.7-.delta. and PrBa.sub.2 Cu.sub.3 O.sub.7-.delta., is disclosed in a paper by Hisamaru Bando, Takahito Terashima, "Applied Physics", volume 60, No. 5 (1991), pp. 474-477.
Also, the Josephson composite fabrication technology of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. /PrBa.sub.2 Cu.sub.3 O.sub.7-.delta. /YBa.sub.2 Cu.sub.3 O.sub.7-.delta. is disclosed in a paper by Jiro Yoshida, Tatsunori Hashimoto, "Applied Physics", volume 61, No. 6 (1992), pp. 494-497.
Further, a superconductive tunnel junction shows a quantum effect and becomes a potential candidate in applying the quantum phenomena in an electronic device. Conventionally, a tunnel type Josephson device has been formed by using superconductive materials of Pb, Nb, NbN etc. As tunnel barriers, PbO, Nb.sub.2 O.sub.3 etc. which are oxides of superconductive materials, or substances different from superconductive materials, for example, Al, Al.sub.2 O.sub.3, MgO, SiO.sub.2 etc. have been used. Concerning a tunnel type Josephson device having such a material composition, excellent properties are provided and application thereof to a superconductive memory, a logic circuit, a magnetic field sensor and millimeter wave detection has been progressed. However, the critical temperature of superconductivity in such a material group is as low as 15 K or less, cooling to an extremely low temperature is necessary, the cost required in the cooling is increased, the facility is magnified and therefore, it is difficult to simply use the tunnel type Josephson device.
In the meantime, superconductive materials having the critical temperature of superconductivity of a liquid nitrogen temperature or higher have been developed in oxide superconductors and the like whereby the materials can be utilized by an inexpensive cooling means. Hence, researches aiming at tunnel type Josephson devices have been progressed. Especially, many researches have been carried out on laminated layer thin films also in view of revealing the mechanism of superconductivity since the crystal structure per se is lamellar in oxide superconductors having high Tc, an intrinsic junction property is observed and so on.
However, with respect to Josephson devices using artificially fabricated oxide superconductive materials, although there have been reports on fabrication thereof of a weak bonding type where controlling of properties is difficult, there has been no report on fabrication thereof in a tunnel type where controllability is excellent. It is conceived that the superconducting coupling between -superconductive electrodes becomes weak due to a deterioration by interdiffusion within the barrier layer since the fabrication temperature of oxide superconductive thin films is as high as 600.degree. C. and the coherent length is short.
It is advantageous in view of crystal growth to combine the same kind of materials for realizing a laminated layer structure. Therefore, in respect of superconductors having a 123 structure with a representative example of YBa.sub.2 Cu.sub.3 O.sub.7-.delta., PrBa.sub.2 Cu.sub.3 O.sub.7 showing no superconductivity despite approximately the same crystal structure, is selected and many fabrication results have been reported. However, in almost all of the conventional cases, YBa.sub.2 Cu.sub.3 O.sub.7-.delta. of which high quality thin film stacking process has been established is selected and fabricated as a superconductor layer.
In the case of YBa.sub.2 Cu.sub.3 O.sub.7-.delta., many screw dislocations due to misfit strain in respect of a substrate are observed, causing problems in the surface flatness as well as the stability. In addition thereto, the lattice match performance in respect of PrBa.sub.2 Cu.sub.3 O.sub.7 is not so excellent. It is known that according to an oxide superconductor, the oxygen depletion amount is increased by the lattice strain whereby the superconductive property is deteriorated. Actually, the critical temperature Tc of an a-axis oriented film on a substrate of SrTiO.sub.3 single crystal is restrained to a low value by the effect of strain. In this way, stresses are accumulated at the inside of a multilayered film of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. and PrBa.sub.2 Cu.sub.3 O.sub.7 and accordingly, high quality superconducting properties are difficult to achieve.
The inventors have found the following problems as a result of investigating the conventional technologies.
Many screw dislocations due to strains are observed in the conventional material of YBa.sub.2 Cu.sub.3 O.sub.7-.delta., thereby causing a problem in the surface flatness that is an indispensable condition for realizing a multilayered film. In addition thereto, the lattice match performance thereof in respect of PrBa.sub.2 Cu.sub.3 O.sub.7-.delta. is not so excellent. It is known that in an oxide superconductor the oxygen depletion amount is increased by lattice strain whereby the superconductivity is deteriorated. Actually, the critical temperature Tc of the a-axis oriented film on the substrate of SrTiO.sub.3 single crystal is restrained at a low value by the effect of strain. In this way, stresses are accumulated at the inside of the multilayered film of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. and PrBa.sub.2 Cu.sub.3 O.sub.7-.delta. whereby high quality superconductivity is difficult to attain.
That is, an oxide superconductor is provided with a low concentration of carriers and accordingly, electric properties thereof are strongly dependent on the content of oxygen supplying hole carriers. In the meantime, the oxygen amount is closely related to the lattice strain. Accordingly, the oxygen depletion is caused under a state where the strains are applied whereby the superconductive property is deteriorated.
Meanwhile, when different materials are combined, internal strain is generally caused from bonded interfaces due to a difference in lattice constants or thermal expansion coefficients. Thereby, not only the mechanical strength is weakened but the superconductive property is deteriorated in the oxide superconductive material. Hence, the inventors found that it was important in realizing the superconductivity having high function to reduce as small as possible the strains in making such a laminated structure.
It is an object of the present invention to provide a superconductive multilayered thin film structure having high quality dispensing with internal stresses since it is constituted by layers having almost perfect lattice matching.
It is an object of the present invention to provide a superconductive wiring where a large current can be made to flow.
Further, it is another aim of the present invention to provide a Josephson device by selecting a superconductor, for example, NdBa.sub.2 Cu.sub.3 O.sub.7 having a lattice constant that is approximately equal to that of a PrBa.sub.2 Cu.sub.3 O.sub.7 layer of a non-superconductor centering thereon thereby realizing a laminated layer structure having low internal stress and accordingly, constituting the high critical temperature Tc thereby providing a tunnel type property by an oxide superconductor thin film.